The present invention relates to a process for the continuous production of elastomer-modified high-impact monovinylaromatic polymers.
This invention is directed to a continuous process for the manufacture of elastomer-modified monovinylaromatic polymers such as high impact polystyrene (HIPS), and more particularly discloses processes and apparatus for the efficient and economical manufacture of HIPS using a unique primary reactor design to replace the conventional pre-inversion reactor (PIR) to allow better temperature control and improved control of rubber particle size (RPS) of the finished product.
The continuous process for making HIPS material consists of polymerizing styrene monomer in the presence of dissolved rubber. Polystyrene is initially formed from the styrene monomer within the homogeneous rubber solution. The usual rubber types utilized in the manufacture of HIPS include polybutadiene (PB), styrene-butadiene rubber (SBR), and styrene-butadiene-styrene rubber (SBS). Current processes for making HIPS materials most generally utilize two continuous-stirred tank reactors (CSTR) in the initial manufacturing process to control grafting of the elastomer and styrene particles as well as controlling the RPS character of the material. One example of the prior art disclosing the use of CSTR reactors is the patent to Echte, U.S. Pat. No. 4,567,232.
Another patent teaching the continuous manufacture of HIPS, which is an improvement over the above-noted Echte patent, is that granted to Sosa, et al, U.S. Pat. No. 4,777,210, the specification of which is incorporated herein by reference. In the Sosa patent, the first CSTR, which is characterized as the PIR portion of the process, is run at conditions which maintain the reacting solution at a point prior to the rubber/styrene inversion point, i.e., the point at which the solution being reacted goes from polystyrene particles in a rubber/styrene monomer matrix to rubber particles in a polystyrene matrix. The inversion point is achieved in the process taught by the Sosa patent in the second CSTR vessel. The Sosa patent teaches that one key element for determining the inversion point is the viscosity of the reaction solution.
Other related patents disclosing the manufacture of HIPS materials include European Patent Specification, publication number 0 254 304 B1, indicating the inventor as Morita, et al, filed Jul. 22, 1987. This patent is directed to controlling RPS so that it exhibits a narrow distribution. This is achieved using multiple tubular reactors containing static mixers.
The U.S. patent to Miura et al, U.S. Pat. No. 5,194,525, discloses a continuous process for making a polystyrene for injection molding purposes, which is made from styrene monomer and a polymerizable unsaturated fatty acid, using a tubular reactor with fixed plural mixing elements formed on the inside thereof.
U.S. Pat. No. 4,521,569 to Bubeck et al, teaches the manufacture of HIPS materials having improved stress crack resistance wherein the polystyrene is polymerized using an aliphatic hydrocarbon as a polymerization solvent rather than an aromatic or cycloaliphatic hydrocarbon. Other patents teaching the manufacture of HIPS materials include U.S. Pat. No. 3,658,946 to Bronstert; and U.S. Pat. No. 4,144,204 to Mittnacht, et al.
European Patent Application No. 0 400 479 filed May 23, 1990, and assigned to Montedipe S.r.l., discloses a process of manufacturing HIPS materials wherein a downflow configured plugflow reactor having reaction zones is used when polymerizing monovinylaromatic monomers with rubber, a vinyl-cyanide comonomer, and a diluent.
Two patents to Cantrill and Doyle, assigned to Novacor Chemicals (International) S.A. of Fribourg Switzerland, U.S. Pat. No. 5,551,859 and Canadian 2,136,655, disclose the process of converting a stable partially polymerized syrup comprising dispersed rubber-like composite particles in a resin phase into a metastable syrup in which the rubber-like composite and resin phases are co-continuous, by subjecting the stable syrup to a high shear under pressure.
One of the critical problems of the above-described prior art processes for manufacturing HIPS materials involves the ability to control grafting and RPS distribution in the first reactor. The prior art processes generally all utilize PIR type reactors to control these properties. Because the reaction occurs so slowly in the first reactor in order to maintain low conversion, the heat of reaction is insufficient to maintain the reaction and a preheater must be utilized to add heat to the solution in order to maintain good temperature control. This is especially true when HIPS manufacturing processes utilize feed solutions having less than five percent by weight of elastomer contained in them.
This addition of heat through a preheater causes another problem to occur in conventional processes. That problem with the prior art process designs is that many of them must utilize internal heat exchanger tubes in one of the secondary reactors in order to remove the excessive heat of reaction from the reactor that occurs due to the added heat that is initially put into the reaction with the preheater.
The present invention overcomes these problems by providing a process which greatly reduces and in most cases eliminates the need for preheaters and internal tube heat exchangers in the polymerization process.
The present invention overcomes the disadvantages of the prior art HIPS manufacturing processes by replacing the normal CSTR type of initial reactor used as a preinversion stage reactor, with a longer and narrower stirred reactor having a much larger Height to Diameter (H/D) ratio than the conventional PIR type of reactor.